Compensated humidity-measuring circuit



Feb. 17, L. COMPENSATED HUMIDITY-MEASURING CIRCUIT Filed Sept. 5, 1950 zo-Powm INVENTOR. LEO S. CRAIG BY ,iVm/S @ja/m f1 fa/wey mmr Patented Feb. 17, 1953 UNITED STATES PATENT O F F l C E COMPENS-A'IED HUMIDITY-MEASURING CIRCUIT (Granted under Title 35, U. S. Code (1952),

sec. 266) 9 Claims.

The invention described in the following specication and claims may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to improvements in compensated humidity-measuring circuits, and more particularly to those designed for use in balloonbo-rne radiosondes.

In the use of a conventional radiosonde transmitter wherein the reference audi-frequency of a relaxation oscillator or audio section is determined by or is dependent upon the value of a fixed resistor connected across a condenser; the latter, after building up a charge sufficient to impart a cut-off bias to the grid of the oscillator tube, discharges through the iixed resistor and a conventional humidity element in the form of an electrolytic-type resistor connected in series with the fixed resistor. responsive structure of the humidity element varies with the ambient relative humidity, and since this variation effects a corresponding change in the audio-frequency of the relaxationoscillator tube, a means is available for quickly and accurately determining the relative humidity. These electrolytic-type resistors or elements are quite reliable in their indications when used in circuits where an alternating-current power supply is available, but when used in a balloonborne radiosonde Where recourse must be had to circuits employing direct-current energy, polarization takes place. To this can be attributed the fact that relatively large errors have been introduced in the data recorded at the meteorological receiving station.

Polarization of the conventional electrolytictype humidity element is generally believed to arise from the fact that the conduction phenomena occurring in the passage of an electric current through a layer of hygroscopic material are essentially electrolytic in nature, and are accompanied by changes at the electrode interface where the current enters the hygroscopic medium, thereby developing a varying counter-electromotive force as well as diminishing the effective interface area. The effect is cumulative .and hence, while a number of hygroscopic-responsive resistors may possess identical characteristics at the moment of impression of an electric voltage thereon, yet, due to variation in their respective polarization characteristics as time passes, the divergent characteristics become increasingly obvious. Polarization eects are very difficult to render Uniform, require correction by a factor controlled by the time of application of current to the resistor, and vary quite widely with temperature.

It has been proposed to prevent, or at least to retard the observed deterioration or poloriza- Since the hygroscopiction of the layer of hygroscopic material in conventional humidity elements, by periodically reversing the current therethrough. A system or arrangement for this purpose is disclosed in Patent No. 2,444,111 issued June 29, 1948, to Charles B. Pears, Jr. In this system mechanical means are employed for switching the poles of the humidity element, thereby to reverse the direction of current-flow through the layer of hygroscopic material. However, in this proposed system, the switching rate is at the very low frequency of approximately two times per minute, and while polarization is retarded, it inevitably takes place.

With the foregoing in mind, it is one of the objects of this invention to provide an improved humidity-measuring circuit of the character referred to wherein provision is made to eliminate errors which otherwise would occur due to polarization elfects attributable to the use of directcurrent energy in the measuring circuit.

Another object resides in the provision of an improved humidity-measuring circuit of the character referred to which has advantages over those used heretofore in the way of increased sensitivity at low temperatures.

Still another object resides in the provision of an improved humidity-measuring circuit of the character referred to wherein there occurs reversal of current thro-ugh the humidity element by electrical means having no mechanical moving parts, and wherein the rate of reversal is approximately one hundred times per second to eliminate any deterioration or polarization whatsoever.

Other objects and advantages will hereinafter appear.

For the purpose of illustrating the invention, an embodiment thereof is shown in the drawing,

wherein l Figure l is a simplified, diagrammatic view of the audio section in a compensated humiditymeasuring circuit constructed and having operating characteristics in accordance with the invention; and

Figure 2 is a View similar to Figure l, showing a conventional form of audio section in which there can occur the disadvantageous operating elects which the improved circuit in Figure l eliminates.

With reference iirst to Figure 2, the numeral l0 designates the relaxation-oscillator tube in a conventional laudio section of a radiosonde transmitter.- In operation, the audio-frequency of tube l0 at any instant is determined by the resistancevalue at that instant of a humidity element l l which, for the purposes of illustration, can be considered to be of the same construction and to have the same intrinsic and operating characteristics as the humidity-responsive resistor shown and described in Patent No. 2,481,728 issued September 13, 1949, to Alexis B.

Dember. In series with electrolytic resistor element II is the usual, Xed, reference resistance I2. These two series-connected resistances II and i2 are connected, as shown, across a con- -denser I4. Oscillation of the circuit causes condenser i!! to build up a charge rapidly, and to a point Whereat the oscillation of the tube is cut off. During this condenser-charging phase of the operating cycle, the direction of current-flow through the various connections and parts is as indicated by the solid arrows. Oscillation of tube` I being cut off, condenser I4 then discharges to ground through the fixed resistance I2 and thence through the interface surface II of resistance element l I, until the tube can once again begin oscillation. During this condenser-discharging phase o-f the operating cycle, the direction of curent-ow through the various connections and parts is as indicated by the broken arrows.

From. the it will be apparent in operation the conventional circuit shown in Figure 2, all ovv or' current through the h osccpic material or inter-Yacc surface ii in the same direction, indicated by the arrows. As explained in the Dember patent aiorec such unidirectional iloiv oi' current results, in time, in appreciable changes and c rre ponrling polarization of the surface I I. To this phenomenon is attri'auted directly the fact that in a conventional circuit such as shown in E-iigure the characteristic of the humidity element Si varied t low temperatures. Any slight, undesirable 'variation in the characteristic of resistance element li causes a correing undesirable change in the audiofreduency of tube Eil, such varia or change becomes apparent in relatively lai errors introduced in the recorded at the rneteorological r-.ceiving station.

Examples. of rc onde a aratus or systems in which the present improved humidity-measuring circuit might be embodied, are illustrated and described in Department or the .army Tecr-u nical Manual iden fl as Till/.i ,1l-.243@ printed by the Unite-i Statesl Government Printing Office, October 1947.

ln the embodiment ci closed in Figure 1, the various parts therein which correspond to or are the equivalent of those in Figure have been designated hy the same respective r r numerals, hut those in "igure i have been given the suiiix rl.. Ti series with the humidity el Elfi. is an av '.ional or auxiliary condenser C, with a discharge resistance-path through` negative tcmperature-scnsitive resistor lf desired, Rt may be composed of one er clore resistors.

.At any given tenul ature, the resistance of humidity element tia is much smaller at high humidities than at low humidities. At low humidity, the interface surface iia of electrolytic resistance iid. has resistance value sufficiently iig-h to constitute practically an open circuit. Under such conditions, the frequency is therefore determined by condenser and resistor Rt, the value of the latter being chosen to give a high frequency wi ich, at the receiving station, would be located about one hundred divisions to the right on the chart paper. At high humidity, the interface suriace iIc of element ia has a resistance velue suiiic'ently low to ccnstitute practically a c "nection placing condensers ltd and C in paral el. `'vl/ith the capacity of condenser C chosen to he ten to twenty times that suhicct invention disand cf condenser I da, it will then take the tivo parallel-connected condensers a much longer time to charge, and then to discharge through resistor Rt, resulting in a loiv frequency. In other Words, low resistance of element llc. results in low frequency, and high resistance of the same results in high frequency.

In operation of the improved circuit in Figure 1, the auxiliary or supplementary condenser C accumulates a charge as condenser iria simultaneously builds up a charge to a point Whereat the oscillation of tube lila is cut oif, as in Figure 2. During this condenser-charging period of the operating cycle in Figure l, the direction of current-now through the various connections and parts is as indicated by the solid arrows. Gscillation of tube lila being cut off, the condensers C and Ida then. discharge to ground through the resistor until tube lila can once again begin oscillation, similarly to the operating action in Figure 2. During this condenser-discharging period of the operating cycle in Figure 1, the direction of current-now through the various connections and parts is as indicated by the broken arrows, condenser C discharging through resistor and condenser lcdischarging through resistor Rt and humidity element Ilo. irorn this it will be seen that during the charging or condensers ii-ic C, the new of current through element iid is in but one direction as indicated by the associated, solid arrow I5. During the discharge period of condensers Ita and C, the current-flow through element iid is also unidirectional as indicated. by the associated, broken arrow l-5, `but is flowing in a direction which is opposite to what it was during the charging period. The effect thus obtained is the same as though an alternating-current power supply were being used, and accordingly, polarization or deterioration of the hyeroscopic material IIa is entirely eliminated.

By choosing for condenser C a value at least ten times that of condenser its, there is obtained the desired range in recorder divisions, for change in the humidity element lla from 100% to 15%.

By choosing for the reference resistor Hic a value of approximately dificil ohms, there results an audio-frequency of 199 cycles which is set on the recording apparatus at 95 divisions (l division-:2 cycles).

An important operating feature in the improved circuit as shown in Figure l, resides in improvement of sensitivity at the low-temperature end of the operating range in actual use or a radiosonde. This is accomplished by appropriate variations in the resistor Rt. For example, an increase in the resistance of humidity element lic results in increased frequency, whereas an increase in the resistance (discharge resi-stance) of resistor Rt results in decreased frequency. if, therefore, an increase ila, due to decrease in temperature at constant humidity, is compensated for by an appropriate increase in resistor Rt, the circuit in Figure l will be temperaturecompensated. To this end, it is proposed to use for Rt a commercially-available resistor made oi semi-conductor material, and having the characteristic desired. These resistors or thermal elements are referred to in the trade as negative temperature coeiiicient resistors, and are available in a variety of resistance values and coefficients.

The values of resistance and capacity given in the drawing are representative, and are given as approximations to obtain the desired results. These values are not critical in any strict sense of the word, and .nay vary over a substantial range, be ter to suit particular requirements.

Various modifications oi the disclosed. embodiment of the invention are possible without departing from the spirit thereof or the scope oi the claires.

What is clair-.ieri ist l. The co.. bination with a compensated humidity-measuring circuit of the character described and comprising a relaxation-oscillator tube having a grid, a resistor and a condenser connected to said grid to determine the frequency oi operation of said tube, said resistor serving as a discharge path for said condenser; of a second condenser connected to said grid to charge and discharge simultaneously with said :first-named condenser, and a humidity element connected in series with said second condenser.

2. The combination with a compensated humidity-measuring circuit of the character described and comprising e, relaxation-oscillator tube having a grid, a resistor and a condenser connected to said grid to de mine the frequency oi. operation of said tube, said resistor serving as a discharge path for said condenser; of a second condenser connected to said grid to charge and dis-charge simultaneously with said first-named condenser, and a humidity element connected in series with said second condenser, the capacity oi said second condenser being at least several times that of said inst-named condenser.

3. In a compensated humidity-measuring circuit of the character described, a relaxationn oscillator tube having a grid, a resistor and a condenser connected to said grid to determine the frequency oi operation oi said tube, said resistor serving as a discharge path for said condenser, a second condenser connected to said grid to charge and discharge simultaneously with said nrstnamed condenser, and a humidity element connected in said circuit and including hygroscopicresponsive structure in series with said second condenser.

4. The combination with a compensated humidity-measuring circuit of the character described and comprising a relaxation-oscillator tube having a grid, a resistor and a condenser connected to grid to determine the frequency `of operation oi said tube, said resistor serving as a discharge path for said condenser; of a second condenser connected to said grid to charge and discharge simultaneously with said first-named condenser, and a humidity element connected in series with said second condenser, the capacity of said second condenser being at least ten times that oi said first-named condenser.

5. In a compensated humidity-measuring circuit of the character described, a relaxationoscillator tube haring a grid, a resistor and a condenser connected to said grid to determine the frequency of operation of said tube, said resistor serving as a discharge path for said condenser, a

second condenser connected to said grid to charge and discharge simultaneously with said rstnamed condenser, and a humidity element connected in said circuit and including hygroscopicresponsive structure in series with said second condenser, said resistor being characterized by the fact that the temperature coefficient thereof is negative to effect a temperatureompensating action at the low-temperature end of the operating range of said circuit.

6. The combination with an oscillatory circuit embodying a relaxation-oscillator tube having a grid, a humidity element and a condenser connected in parallel relation and to said grid to cause charging of said condenser during one interval of each oscillation of said tube and otherwise to cause discharge of said condenser through said element during another interval of each oscillation of said tube; of a second condenser seriesconnected with said `element and in said circuit to charge simultaneously with said first-named condenser and to discharge simultaneously with the latter thereby to block current-flow through said element from said nrst-named condenser during said discharge of the latter.

'2. The combination with an oscillatory circuit embodying a relaxation-oscillator tube having a grid, a humidity element and a condenser connected in parallel relation and to said grid to cause charging of said condenser during one interval of each oscillation of said tube and otherwise to cause discharge of said condenser through said element during another interval of each oscillation of said tube; of a second condenser seriesconnected with said element and in said circuit to charge simultaneously with said first-named condenser, and a resistor connected in said circuit to provide a common discharge path :for both of said condensers, said second condenser being disposed between said element and the high-potential side of said first-named condenser to block current-now through said element from said firstnamed condenser during said discharge of the latter.

8. The combination with an oscillatory circuit embodying a relaxation-oscillator tube having a grid, a humidity element and a condenser connected in parallel relation and to said grid to cause charging of said condenser during one interval of each oscillation of said tube and otherwise to cause discharge of said condenser through said element during another interval of each oscillation of said tube; of a second condenser in series with said element to prevent polarization of said element by balancing current-flow through said element during said intervals.

9. The combination with an oscillatory circuit embodying a relaxation-oscillator tube having a grid, a. humidity element and a condenser connected in parallel relation and to said grid to cause charging of said condenser during one interval of each oscillation of said tube and otherwise to cause discharge of said condenser through said element during another interval of each oscillation of said tube; of a second condenser in series with said element to prevent polarization of said element by balancing current-flow through said element during said intervals, and a resistor connected in said circuit to provide a common discharge path for both of said condensers.

LEO S. CRAIG.

REFERENCES CITED The following references are of record in the 

